Multicarrier transmission method and apparatus

ABSTRACT

A multicarrier transmission apparatus that achieves both power efficiency in a high-frequency amplifier and quality of transmission signal in a radio transmission system in next-generation mobile communications that supports high speed data packet transmission. In the apparatus, adaptive peak limiter  400  performs adaptive hard limit processing on baseband signals of frequency channels passed through multiplexing sections  200   a  to  200   d  for users, multicarrier signal generating circuit  500  with peak suppressing function performs peak limit processing on a synthesized multicarrier signal, hybrid distortion compensating circuit  700  corrects non-linear distortion due to high-frequency amplifier  32 , and the multicarrier signal is transmitted from an antenna (ANT).

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a multicarrier transmission method andapparatus, and more particularly, to a method of suppressing a peak of amulticarrier transmitted signal, multicarrier transmission signalgenerating circuit with peak suppressing function, adaptive peaklimiter, baseband signal processing LSI and multicarrier transmissionapparatus.

2. Description of Related Art

In the field of mobile communications, for example, with respect totechnical specifications in the W-CDMA (Wideband-Code Division MultipleAccess) system, 3GPP has standardized. In the technical specifications,in addition to basic reception techniques (for example, Rake combining)that take advantages of CDMA, High Speed Downlink Packet Access(hereinafter, also abbreviated as HSDPA) with a faster rate of 10 Mbpshas been also standardized.

HSDPA is a technique where a plurality of users that perform downlinkpacket transmission shares a downlink channel, radio channel quality ofeach user is checked, and optimal base stations transmit signals torespective users, thus improving the transmission efficiency. Using thetechnique achieves a transmission rate of 10 Mbps using a frequencybandwidth of 5 MHz.

Among specific techniques used in HSDPA are adaptive modulation that isa scheme for varying a modulation scheme and coding scheme correspondingto propagation environments and using M-ary modulation such as 16QAM and64QAM suitable for large-capacity transmission, HARQ (Hybrid ARQ) thatsynthesizes a retransmission signal to improve the reception quality,and FCS (Fast Cell Selection) that achieves efficient packettransmission from a plurality of base stations.

In adaptive modulation, a base station transmits signals with M-arymodulation such as 16QAM and 64QAM and high-rate coding with a codingrate of, for example, ¾ when the reception quality in a mobile stationis good, while transmitting signals with QPSK and low-rate coding with acoding rate of, for example, ¼ when the reception quality in a mobilestation is poor.

In HSDPA, since QAM is used as a modulation scheme, with respect tosignal quality in a band (Peak Code Domain Error (PCDE) and Error VectorMagnitude (EVM)), it is mandatory to comply with performance standard(TS25.141 Rel.5) stricter than performance standard (TS25.141 Rel.99)for general third-generation base station apparatus.

Meanwhile, the CDMA system has a significant feature of implementingconcurrent communications by multiplexing user signals. For example, itis assumed that a frequency band assigned to a company permitted tolocate base stations includes four channels (with carrier frequencies f1to f4 respectively).

In this case, data of a plurality of users is multiplexed on onechannel, and signals of the channels are transmitted at the same timefrom a shared antenna. In other words, four carriers, f1 to f4, areconcurrently transmitted (multicarrier transmission).

When the multicarrier transmission is performed, a high-frequencyamplifier provided at a last stage of a transmitter undergoes a heaveload, and is required to secure the linearity in a wide band.

In order to reduce the load on the high-frequency amplifier, using apeak limiter, the processing for suppressing an instantaneous peak isperformed on a baseband signal for multicarrier transmission.

The peak limiter is described in, for example, Japan Laid-Open PatentPublication Nos. 2002-164799 and 2002-44054.

However, conventional techniques related to the peak limiter have noconsideration on High Speed Downlink Packet Access (HSDPA).

HSDPA is an advanced technique with a considerable amount of difficultyto practically implement, in any theory.

Further, as described above, since QAM is used as a modulation scheme,with respect to signal quality in a band (PCDE and EVM), HSDPA needs tomeet performance standard (TS25.141 Rel.5) stricter than performancestandard (TS25.141 Rel.99) for general third-generation base stationapparatus.

In mobile communication apparatuses such as cellular telephones, therehave been severe demands always for cost reduction, miniaturization andlow power consumption.

It is difficult for conventional techniques to implement HSDPA undervarious constrains imposed on mobile communication apparatuses.

For example, in HSDPA, the modulation scheme is varied corresponding tothe channel quality. In this case, when characteristics of a peaklimiter are adapted to 64QAM with the strictest conditions as areference and peak suppression is reduced, since the suppression on aninstantaneous peak is not sufficient, and as a result, the load on ahigh-frequency amplifier in a subsequent stage is increased and thepower efficiency in the high-frequency amplifier deteriorates.Meanwhile, when the peak suppression is enhanced, a signal loss degradesthe signal quality.

In order to solve the problem, it is necessary to use an amplifier withhigh performance that secures the linearity in an extremely wide range.However, such a high-frequency amplifier is expensive, and becomes asignificant obstacle in cost.

The problem is described above referring to W-CDMA communications as anexample, but there is a possibility such a problem occurs in othercommunication systems (such as other CDMA systems) that performhigh-speed packet transmission.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a multicarriertransmission method and apparatus that take realistic measures ontechniques of limiting a peak and compensating for a distortion of atransmission signal, while clearing up strict constrains imposed onmobile communication apparatuses, and enable desired characteristics ofthe transmission signal to be achieved.

According to an aspect of the invention, a method of suppressing a peakof a multicarrier transmission signal, in a transmission system wherefiltering processing is performed on each of baseband signalsrespectively corresponding to a plurality of frequency channels using afilter, the signals subjected to the filtering processing are eachmultiplied by a predetermined carrier to be single-carrier signals, andthe single-carrier signals are combined to obtain a multicarriertransmission signal, has the steps of branching each of the basebandsignals from a regular signal processing route, performing filteringprocessing on each of the baseband signals branched, multiplying each ofthe baseband signals branched by the same carrier as the predeterminedcarrier at the same timing as in multiplication by the predeterminedcarrier, combining the signals obtained, and thereby obtaining amulticarrier signal for use in calculating a correction value for peaksuppression, detecting an instantaneous peak of the multicarrier signalfor use in calculating the correction value, and based on the detectionresult, obtaining the correction value for peak suppression, andmultiplying each of the baseband signals on the regular signalprocessing route by the correction value to perform correction for peaksuppression.

In other words, in the method of suppressing a peak of a multicarriertransmission signal of the present invention, a single-carrier signal issynthesized obtained by performing the same processing under the samecondition as in the processing in the regular signal route, the samemulticarrier signal as the regular multicarrier signal is obtained, acorrection value is calculated based on the obtained signal, and usingthe correction value, an amplitude value of a baseband signal of eachfrequency channel is corrected.

Such a method is equivalent to accurately predicting an instantaneouspeak that will occur in synthesizing a single-carrier signal, andcorrecting the amplitude of a baseband signal in advance so as to keepthe instantaneous peak within a desired level. In this way, it ispossible to perform a method of suppressing a peak with extremely highreliability that has not been obtained before.

Further, in an aspect of the method of suppressing a peak of amulticarrier transmission signal of the present invention, even when thepeak of a baseband signal decreases continuously, the correction valueis calculated using a high peak value obtained before the peak startsdecreasing in a predetermined number of times, enabling severer peaklimitation.

Thus, it is possible to perform adaptive control with more importanceplaced on peak suppression than signal quality, rather than mereadaptive control. In other words, in any cases, it is possible tosuppress an amplitude value of a multicarrier signal (combinedtransmission single-carrier signals) to be below a desired level.Accordingly, the load on a high-frequency amplifier disposedsubsequently is always reduced.

According to another aspect of the invention, a multicarriertransmission signal generating circuit with peak suppressing functionhas a regular signal processing route for branching each of basebandsignals corresponding to each of frequency channels tomulticarrier-transmit to two signal sequences, delaying each of basebandsignals in one signal sequence in a delayer, multiplying each of thesignals by a correction value for peak suppress in a multiplier,performing n-time (n is an integer of two or more) interpolationprocessing on each of the signals multiplied by the correction value,performing filtering processing on the signals using a filter,multiplying each of the signals by a carrier to obtain single-carriersignals, and combining the single-carrier signals to output amulticarrier transmission signal, and a correction value generatingroute for performing on each of baseband signals in the other signalsequence substantially the same processing at substantially the sametiming as the n-time interpolation processing, the filtering processing,and processing of multiplying by the carrier to obtain a single-carriersignal in the regular signal processing route, thereby obtaining amulticarrier signal for use in calculating the correction value,detecting an instantaneous peak of the multicarrier signal for use incalculating the correction value, and obtaining the correction value forpeak suppression based on the detection value to provide to themultiplier in the regular signal processing route.

In other words, in the multicarrier transmission signal generatingcircuit with peak suppressing function of the present invention, aninstantaneous peak is detected based on a multicarrier signalsynthesized through a route with the completely same conditions as inthe regular multicarrier synthesis route to calculate the correctionvalue for peak suppression, and it is thereby possible to performextremely accurate peak suppressing correction.

According to still another aspect of the invention, an adaptive peaklimiter has a plurality of hard limiters which is provided respectivelyfor a plurality of frequency channels having a possibility of containingcommunication data to which a predetermined data packet transmissionscheme is applied, and limits an amplitude value of a baseband signal ofeach of the frequency channels using an adaptive limit value providedfrom outside, and a limit value table to which access is made using, asan address variable, on/off bit information indicative of whether thepredetermined data packet transmission scheme is applied and anotheron/off bit information indicative of whether each of the frequencychannel is used, both the information being reported from an upper layerfor each of the frequency channels, and which outputs an adaptive limitvalue as a result of the access to provide to at least one of theplurality of hard limiters.

In other words, the adaptive peak limiter of the present invention is anew peak limiter contributing to implementation of, for example, HighSpeed Downlink Packet access (HSDPA) supported by 3.5-Generation MobileCommunications.

The adaptive peak limiter of the present invention has a plurality ofhard limiters which is provided respectively for frequency channels andwhose limit values are updated adaptively. The “hard limiter” is alimiter with the capability of clamping a peak value of a signal in apredetermined value precisely.

In an aspect of the adaptive peak limiter of the invention, as addressinformation, using on/off information indicative of whether or not eachfrequency channel is used and another on/off information indicative ofwhether or not HSDPA is applied to chip data of each frequency channelnotified from an upper layer (for example, a baseband processing boardin a base station control section) on a chip basis of a baseband signalcorresponding to each frequency channel in multicarrier transmission,the limit value table is accessed to output a limit value adaptively,and a clamp value of the hard limier is thereby adjusted finely on achip basis.

That is, concurrently transmitting signals of a plurality of frequencychannels does not include using all the frequency channels always, andeven when a chip of a transmission signal of a frequency channel is chipdata using HSDPA (using QAM as a modulation scheme), chip data of theother frequency channels to concurrently transmit does not use HSDPAalways (in other words, QPSK may be used as a modulation scheme).

With attention attracted to this respect in the present invention, theadaptive control is performed such that on a chip to which a modulationscheme with severe demodulation condition such as 16QAM is applied withfrequency channel ON, the adaptive control with relieved suppression onamplitude value and importance placed on signal quality is performed,while when a frequency channel is OFF or on a chip to which HSDPA is notapplied, adaptive control with enhanced suppression on limit value andimportance placed on peak suppression is performed.

Thus, in response to a state of each frequency channel, bydistinguishing chip data in which importance should be placed onamplitude suppression and chip data in which importance should be placedon signal quality from one another, it is possible to perform adaptivefine adjustment so as to provide more energy to the chip data withimportance placed on signal quality. Accordingly, in the case where datato multicarrier-transmit includes transmission data using HSDPA, it ispossible to assure desired signal quality specified in specifications of3GPP.

According to a further aspect of the invention, a baseband signalprocessing LSI has a configuration where respective signals of frequencychannels output from the adaptive peak limiter as described above areinput the multicarrier transmission signal generating circuit with peaksuppressing function as described above, thereby generating amulticarrier transmission signal subjected to peak suppressingprocessing such that a PAR (Peak to Average Ratio) value and a CCDF(Complementary Cumulative Distribution Function) remain withinrespective desired allowable ranges.

That is, the baseband signal processing LSI of the present inventionuses the adaptive peak limiter of the invention and the multicarriertransmission signal generating circuit with peak suppressing function ofthe present invention in a combination thereof.

In other words, peak limit processing is carried out in the multicarriertransmission signal generating circuit with peak suppressing function sothat a PAR (Peak to Average Ratio) value and a CCDF (ComplementaryCumulative Distribution Function) of a multicarrier signal always remainwithin respective desired allowable ranges (which assures that the totalenergy of the transmission signal always remains within a predeterminedrange), while using the peak limiter, a limit value is adaptivelycontrolled for each frequency channel, and fine adjustment ondistribution of transmission energy is carried out in response to astate of each frequency channel.

In this way, extremely efficient (rational) adaptive peak limitprocessing is achieved in consideration of both the entire state ofmulticarrier signal and state of each frequency channel. Therefore, withrespect to a plurality of frequency channels having a possibility ofcontaining communication data to which applied is a high speed datapacket transmission scheme (such as HSDPA) conforming to IMT2000,reliable and effective peak suppression is implemented such that the PARvalue and CCDF of a multicarrier transmission signal are kept withinrespective desired allowable ranges to reduce the load on ahigh-frequency amplifier subsequently disposed, while with respect toeach frequency channel, adaptive peak control as possible is performedto prevent signal quality from deteriorating.

According to the present invention, it is possible to achieve latestmobile communications. For example, it is possible to achieve W-CDMAmulticarrier transmission apparatuses in compliance with 3.5-GenerationMobile Communications supporting the HSDPA scheme.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the invention will appearmore fully hereinafter from a consideration of the following descriptiontaken in connection with the accompanying drawing wherein one example isillustrated by way of example, in which;

FIG. 1A is a block diagram illustrating part of an entire configurationof a multicarrier transmission apparatus according to one embodiment ofthe present invention;

FIG. 1B is a block diagram illustrating remaining part of the entireconfiguration of the multicarrier transmission apparatus according tothe present embodiment;

FIG. 2 is a block diagram illustrating one example of a configuration ofa multicarrier signal generating circuit with peak suppressing functionof the present invention;

FIG. 3 is a graph showing CCDF (Complementary Cumulative DistributionFunction) of a multicarrier transmission signal generated in the circuitin FIG. 2;

FIG. 4 is a graph showing characteristics of each single-carrier signalprior to multicarrier synthesis in the circuit in FIG. 2;

FIG. 5A is a view to explain an issue in the case of performing peaksuppressing processing on a single-carrier signal;

FIG. 5B is a view to explain advantages in the case of performing peaksuppressing processing on a single-carrier signal;

FIG. 6A is a block diagram illustrating a configuration of aconventional peak limit circuit that performs limit processing on asingle-carrier signal;

FIG. 6B is a graph showing CCDF characteristics of a single-carriersignal (prior to filtering by a low-pass filter) in the circuit in FIG.6A;

FIG. 7A is a block diagram illustrating a configuration of aconventional peak limit circuit that performs limit processing on asingle-carrier signal;

FIG. 7B is a graph showing CCDF characteristics of a single-carriersignal (subjected to peak suppression and filtering by a low-passfilter) in the circuit in FIG. 7A;

FIG. 8A is a block diagram illustrating a configuration of aconventional multicarrier signal generating circuit with conventionalpeak limit circuits each shown in FIG. 6A arranged in parallel;

FIG. 8B is a graph showing CCDF characteristics of a multicarrier signalin the circuit in FIG. 8A;

FIG. 9 is a graph to explain peak detecting operation in a peakcorrection value calculating section in FIG. 1B;

FIG. 10 is a flowchart illustrating procedures of primary operation inthe peak correction value calculating section in FIG. 1B;

FIG. 11 is a view to explain specific procedures of calculating acorrection value;

FIG. 12 is a view to explain effects by changes in set value of peaksuppressing control parameter;

FIG. 13 is a block diagram illustrating one example of a configurationof an adaptive peak limiter of the present invention;

FIG. 14A is a graph showing output characteristics of a hard limiter;

FIG. 14B is a table showing a relative relationship between a limitvalue of the hard limier and signal quality of a transmission signal;

FIG. 15 is a timing diagram illustrating frequency channel on/offinformation and HSDPA application on/off information associated with abaseband signal;

FIG. 16 is a view to explain the association between ROM address and ROMdata;

FIG. 17A is a graph showing an example of a measurement result of errorvector magnitude;

FIG. 17B is a graph showing an example of a measurement result of a peakcode domain error;

FIG. 18A is a graph showing an example of a result of measuring CCDFcharacteristics of a multicarrier transmission signal using test model 1of 3GPP;

FIG. 18B is a graph showing an example of a result of measuring CCDFcharacteristics of a multicarrier transmission signal using test model 3of 3GPP;

FIG. 19 a block diagram showing an example of a configuration of ahybrid distortion compensating circuit (including a high-frequencyamplifier) used in the present invention;

FIG. 20 is a flowchart illustrating procedures of primary operation inthe hybrid distortion compensating circuit;

FIG. 21A is a view showing an example of a frequency spectrum of amulticarrier transmission signal to input to the hybrid distortioncompensating circuit;

FIG. 21B is a view showing an example of a frequency spectrum of asignal subjected to pre-distortion processing;

FIG. 21C is a view showing an example of a frequency spectrum of astandard signal to input to a feedforward distortion compensatingcircuit;

FIG. 21D is a view showing an example of a frequency spectrum of amulticarrier transmission signal output from the hybrid distortioncompensating circuit; and

FIG. 22 is a graph to explain power efficiency of a high-frequencyamplifier.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

An embodiment of the present invention will be described belowspecifically with reference to accompanying drawings.

FIGS. 1A and 1B are block diagrams illustrating an entire configurationof a multicarrier transmission apparatus according to one embodiment ofthe present invention. The multicarrier transmission apparatus is aW-CDMA radio transmission apparatus (radio base station apparatus) thatuses an adaptive peak limiter and multicarrier signal generating circuiteach according to the present invention further in a combination with ahybrid distortion compensating circuit that performs distortioncompensation with high precision.

In the figure, baseband signal processing section (baseband signalprocessing LSI) 600 is indicated by alternate long and short dashedlines.

Adaptive peak limiter 400 and multicarrier signal generating circuit 500with peak suppressing function are both indicated by dotted bold lines.

As shown in the figure, hybrid distortion compensating circuit 700 has aconfiguration with a combination of adaptive pre-distortion section 14and feedforward distortion compensating section 30. As other structuralelements, the circuit 700 has D/A converter 20, A/D converter 28,switching circuit SW and high-frequency amplifier 32.

The following description is given, assuming that an antenna is capableof transmitting signals of four frequency channels at the same time.

As described at upper left in FIG. 1A, transmission data d1 to d3 ismultiplexed on frequency channel CH1, and similarly, transmission datad4 to d6, d7 to d9 and d10 to d12 is respectively multiplexed onfrequency channels CH2 to CH4.

Multiplexing sections 200 a to 200 d of user signal are providedrespectively for frequency channels CH1 to CH4, and each has a pluralityof spreading sections 2 and multiplexing circuit 4 that multiplexesspread user signals.

Adaptive peak limiter 400 has hard limiters 300 provided for eachfrequency channel and limit value output circuit 350. Limit value outputcircuit 350 has address transforming circuit 352 and limit value table(ROM) 354.

Address transforming circuit 352 transforms on/off information (F1 toF4) indicative of whether or not a respective frequency channel is usedand another on/off information (DP1 to DP4) indicative of whether or notHSDPA is applied to chip data on the respective frequency channelnotified on a chip basis from an upper layer (for example, a basebandprocessing board of a base station control section, not shown) toaddresses for use in referring to ROM, and accesses to limit value table(ROM) 354 to cause adaptive limit value LIM to be output.

Adaptive peak limiter 400 will be described specifically later withreference to FIGS. 13 to 17.

Multicarrier signal generating circuit 500 with peak suppressingfunction performs n-time interpolation (n is an integer of two or more)and orthogonal modulation (by multiplying by respective one oforthogonal carriers e1 to e4 to obtain single-carrier signals) on fourfrequency channels, CH1 to CH4, and combines the single-carrier signalsto generate a multicarrier transmission signal, while generating thesame multicarrier signals as the above multicarrier signal in a routedifferent from a regular signal processing route, calculating acorrection value to correct an instantaneous peak based on themulticarrier signal, sending back the correction value to the regularsignal processing route, and subjecting peak suppressing processing on abaseband signal to a baseband signal of each of the frequency channels.

In addition, a baseband signal of each of frequency channels CH1 to CH4is composed of two signals, I (in-phase) signal and Q (quadrature)signal, but for convenience in drawing, is indicated by a single signalline.

Since a configuration of circuitry for performing n-time interpolationand orthogonal modulation is the same as in each frequency channel, thedescription is given on frequency channel CH1.

The circuitry for performing n-time interpolation and orthogonalmodulation has a route for synthesizing a multicarrier signal to be abase to calculate a peak correction value, calculates the correctionvalue based on the signal to provide to the regular signal processingroute(indicated by bold lines in the figure), in addition to the regularsignal processing route.

The regular signal processing route (indicated by bold lines in thefigure) has first delay circuit 508, multiplier 512 to multiply by acorrection value, n-time interpolation circuit 514, low-pass filter(LPF) 516 to limit a band of a signal, multiplier 518 for orthogonalmodulation, and combiner 590 that combines single-carrier signals passedthrough multiplier 518. First delay circuit 508 delays a signal by atime required for calculating the correction value and a timecorresponding to a group delay of LPF 504. N-time interpolation circuit514 performs interpolation for the need of increasing a clock frequencyso as to perform signal processing for a predetermined wide frequencyband.

The orthogonal modulation is achieved by multiplying each of I and Qsignals of each channel by the carrier e1 (to e4).

For example, in the orthogonal modulation, when frequency channels touse are CH1 (carrier frequency f1) and CH2 (carrier frequency f2) and afrequency for frequency shift is fc, a signal to transmit with thecarrier frequency f1 is multiplied by a carrier of f1-fc, whilemultiplying a signal to transmit with the carrier frequency f2 by acarrier of f2-fc, to perform orthogonal modulation.

The multiplication by the carrier e1 (to e4) is performed in the sameway as in the signal processing route for calculating the correctionvalue. Accordingly, in the circuit in FIGS. 1A and 1B, a signal ismultiplied by the carrier e1 (to e4) in the signal processing route forcalculating the correction value, is delayed by a time required forcalculating the correction value and a time corresponding to a groupdelay of low-pass filter (LPF) 516, and is subjected to orthogonalmodulation in the regular signal processing route.

Meanwhile, in the circuit in FIGS. 1A and 1B, a signal route forsynthesizing a multicarrier signal as a base for calculating thecorrection value is provided separately from the regular signalprocessing route. The signal processing route has n-time interpolationcircuit 502, low-pass filter (LPF) 504, multiplier 506 and combiner 550.A configuration of such a signal route and conditions such as signalprocessing timing in the route are completely the same as in the regularsignal processing route.

Correction value calculating section 570 that calculates a peakcorrection value based on the synthesized multicarrier signal has peakdetecting section 572, relative comparison/determination section 574 andcorrection value calculating section 576.

A peak suppressing control parameter is provided to relativecomparison/determination section 574 from the outside, and it is therebypossible to adjust finely the peak suppressing capability depending onthe relative level of importance between the signal quality and peaksuppression.

A peak limit value is provided to correction value calculating section576 from the outside.

A correction value output from correction value calculating section 576is multiplied by a baseband signal in multiplier 512 in the regularsignal processing route to correct the amplitude.

Thus generated multicarrier signal undergoes distortion compensation inhybrid distortion compensating circuit 700, and then is transmitted to aplurality of mobile terminals (not shown) through an antenna (ANT) ofthe base station apparatus.

The frequency band of a spectrum emission mask specified in 3GPP TS25.104 is an extremely wide band that covers about 1 GHz between theupper and lower sides with a band of a transmission signal as a center.Since it is completely impossible for a general pre-distortion circuitto remove distortion components of the high orders occurring in such awide band, using hybrid distortion compensating circuit 700 enablesresponses to such strict specifications.

It is thereby possible to achieve 3.5-Generation W-CDMA MobileCommunications that supports HSDPA.

The multicarrier signal generating circuit with peak suppressingfunction, adaptive peak limiter and hybrid distortion compensatingcircuit will be described specifically below sequentially.

FIG. 2 is a block diagram illustrating a configuration of themulticarrier signal generating circuit with peak suppressing function.The multicarrier signal generating circuit has the same configuration ascircuit 500 as shown in FIG. 1B.

In FIG. 2, reference numerals 530 a to 530 d are circuits each forperforming orthogonal modulation on a baseband signal subjected toamplitude correction to be a single-carrier signal.

Correction value calculating circuit 570 calculates a correction valuebased on a multicarrier signal synthesized through completely the sameprocessing as in the regular signal processing route.

Thus, single-carrier signals are actually combined, an instantaneouspeak of the actual multicarrier signal is detected (in peak detectingsection 572), and a correction value to suppress the peak to be below adesired level is calculated, whereby it is possible to assuredlysuppress the peak of the multicarrier signal to be within a desiredlevel.

FIG. 3 is a graph of CCDF (Complementary Cumulative DistributionFunction) indicative of the relationship between a ratio (horizontalaxis) of peak power to average power of a multicarrier transmissionsignal generated in the multicarrier signal generating circuit with peaksuppressing function in FIG. 2 and probability (vertical axis).

In the figure, it is understood that a considerably abrupt peak limit ispossible like characteristic line A indicated by solid lines. Thecharacteristic line A is of peak suppressing control parameter(hold-num) of two.

In the figure, characteristic line B is of peak suppressing controlparameter (hold-num) of zero, and characteristic line C is of peaksuppressing control parameter (hold-num) of three.

From FIG. 3, it is understood that using a peak suppressing controlparameter (hold-num) provided to relative comparison/determinationsection 574 from the outside enables adjustment of peak suppressingcharacteristics.

FIG. 4 is a graph showing characteristics of each single-carrier signalprior to multicarrier synthesis in the multicarrier signal generatingcircuit with peak suppressing function in FIG. 2.

Herein, FIG. 7B is a graph showing characteristics of a signal subjectedto peak suppression in the conventional circuit for suppressing a peakof a singe-carrier signal as shown in FIG. 7A. By comparing FIG. 4 withFIG. 7B, it is understood that the circuit in FIG. 2 does not performextremely strict carrier suppression on a single-carrier signal.

In other words, in the circuit in FIG. 2, as shown in FIG. 3, themulticarrier signal undergoes the peak limit to a high extent, and inany circumstances, the instantaneous peak of a multicarrier is assuredto be kept within a predetermined range, while in terms ofsingle-carrier signal, excessive peak limit is not performed, andaccordingly, quality of the transmission signal is affected a little.

As shown in FIG. 5A, when the peak limit is performed on each ofsingle-carrier signals X, Y and Z with different phase and differentamplitude on a phase plane, portions extending from a circle of thelimit value are all clamped. On the contrary, when the peak limit isperformed on a multicarrier signal, as shown in FIG. 5B, since the peaklimit processing is performed using vector R obtained by combiningsingle-carrier signals X, Y and Z as a reference, in terms of eachsingle-carrier signal, excessive peak limit is not performed. Inaddition, as shown in the figure, vector R obtained by combiningsingle-carrier signals X, Y and Z is small in amplitude because vectorcomponents of the single-carrier signals are canceled.

Then, in the circuit in FIG. 2, a multicarrier signal is actuallysynthesized, an instantaneous peak of the signal is detected, and acorrection value is calculated to suppress the instantaneous peak,whereby it is possible to suppress the instantaneous peak assuredly.Since thus peak limit with extremely high reliability is performed formulticarrier signal, it is possible to meet predetermined specificationseven when strict conditions are imposed as in applying the HSDPA scheme.

Further, in the present invention, it is possible to finely adjust peaksuppressing characteristics by setting the carrier suppressing controlparameter (hold-num).

FIG. 6A illustrates a conventional peak limit circuit that performslimit processing on single-carrier signal S1, and FIG. 6B showscharacteristics of single-carrier signal S1.

FIG. 7A illustrates the same conventional peak limit circuit as in FIG.6A, and FIG. 7B shows characteristics of the signal which has undergonepeak suppression and is passed through low-pass filter (LPF) 203.

As shown in FIG. 7B, even when the single-carrier signal undergoes peaksuppression, a peak is generated again by being passed through thelow-pass filter, and the degree of peak suppression is considerably pooras compared with peak suppressing characteristics in the circuit of thepresent invention as shown in FIG. 3.

FIG. 8A illustrates a conventional circuit that synthesizes amulticarrier signal using conventional peak limit circuits each shown inFIG. 6A arranged in parallel, and FIG. 8B is a view showingcharacteristics of multicarrier signal S3 output from the circuit inFIG. 8A.

Referring to FIGS. 9 to 12, the operation will be described of each ofpeak detecting section 572, relative comparison/determination section574 and correction value calculating section 576 in correction valuecalculating section 570 shown on lower side of FIG. 2.

As shown in FIG. 9, peak detecting section 572 detects peak values of(M(n) to M(n+2)) of amplitude of baseband signals of 16 chips (herein,for convenience, baseband signals of 16 chips are assumed to be A(n) toA(n+2)).

Relative comparison/determination section 574 and correction valuecalculating section 576 operate based on the flowchart shown in FIG. 10.

In other words, with respect to peak value Max(n) of amplitude of acurrently measured baseband signal detected in peak detecting section572 (step 800), relative comparison/determination section 574 determineswhether the peak value Max (n) is smaller than a temporal peak value andwhether the peak suppression control parameter (hold-iter=0) is notequal to a set value (hold-num: herein assumed as 2) (step 802).

When the determination result in step 802 is “yes”, in other words, thecurrently detected peak value is smaller than the temporal peak valueand the number of times the value continuously decreases is 1, the peaksuppressing control parameter (hold-iter) is incremented and updated(step 806), while, when the result is “no”, that is, the currentlydetected peak value is larger than the temporal peak value, or decreasescontinuously two times, the currently detected peak value is set as apresent value, and the peak suppressing control parameter (hold-iter) isinitialized and returned to zero.

Then, correction value calculating section 576 compares the limit value(limit-value) with the temporal peak value (step 808), and when thetemporal peak value is larger, calculates a correction value for peaksuppression using the temporal peak value (step 810), while, when thetemporal peak value is smaller, making a correction value “1” becausethe peak suppression is not required (step 812).

The correction value is multiplied by the baseband signal (step 814),and the processing flow proceeds to a next step (step 816).

FIG. 11 illustrates a specific example of calculating a correctionvalue.

As shown in the figure, the peak value increases during a period of fromtime t(n−1) to t(n+1), then decreases continuously, and decreases belowthe limit value (limit-value) at time t(n+5).

In this case, processing as shown in the figure is executed at eachtime. It is noted that at time t(t+2) and t (n+3), even the peak valuedecreases, the correction value based on a large peak value immediatelybefore the peak value starts decreasing, that is, correction value (n+1)is used, and thus adaptive control is performed with importance placedon peak suppression.

Next, at time t(n+4), the peak decreases continuously (three times),exceeding the set value (hold-num=2) of peak suppressing controlparameter. Therefore, in order to prevent signal quality fromdeteriorating, the peak limit is performed using the temporal peak valueand relieved correction value, that is, correction value (n+4).

Then, at time t(n+5), since the peak limit is not required, thecorrection value is “1”.

Increasing the set value (hold-num) of peak suppressing controlparameter results in adaptive control with importance placed on peaksuppression as shown in FIG. 3, and enables fine adjustment.

FIG. 12 shows changes in the degree of amplitude suppression of basebandsignal when the set value (hold-num) of peak suppressing controlparameter is “0” or “2”. In the figure, as can be seen from portions Aand B enclosed by solid lines, the set value (hold-num) of peaksuppressing control parameter of “2” has a larger effect of peaksuppression.

By setting the set value (hold-num) of peak suppressing controlparameter as appropriately, it is possible to suppress peaks of amulticarrier signal to be within a predetermined level in any cases. Itis thereby possible to meet strict specifications assuredly.

The adaptive peak limiter will be described below with reference toFIGS. 13 to 17.

FIG. 13 is a block diagram illustrating a configuration of the adaptivepeak limiter.

As described above, based on the on/off information (F1 to F4)indicative of whether or not a respective frequency channel is used andanother on/off information (DP1 to DP4) indicative of whether or notHSDPA is applied to chip data on the respective frequency channelnotified on a chip basis from baseband control board 910 in base station(BTS) control section 900, limit value output section 350 refers to thelimit value table (lookup table) 354 to cause the limit value LIM to beoutput.

The hard limiter 300 has amplitude calculating section 310 thatcalculates amplitude Xn of each of input I and Q signals, comparingsection 320 that compares the calculated amplitude with the limit valueLIM, correction value calculating section 330 that calculates acorrection value from the input I and Q signals, amplitude Xn and limitvalue LIM, and switch circuits SWT1 and SWT2.

Corresponding to a comparison result in comparing section 320, switchesSWT1 and SWT2 are each switched, and when amplitude of an input signalexceeds the limit value LIM, the switches are switched to respective “a”terminals, while being switched to respective “b” terminals whenamplitude of an input signal is less than the limit value LIM. Whenswitches SWT1 and SWT2 are switched to respective “b” terminals, theinput signal is not corrected and output.

FIG. 14A is a view showing characteristics of an output signal of thehard limiter when the limit value is P0, P1 or P2, and FIG. 14B is aview showing the relative relationship between the limit value andquality of a transmission signal.

The operation of address transforming section 352 and configuration oflimit value table (ROM) 354 in limit value output circuit 350 will bedescribed specifically with reference to FIGS. 15 and 16.

FIG. 15 is a timing diagram illustrating states of baseband signals offour frequency channels CH1 to CH4 (each channel has two signalssequences, I and Q, and thus there are eight inputs) associated withsates of the on/off information (F1 to F4) indicative of whether or nota respective frequency channel is used and another on/off information(DP1 to DP4) indicative of whether or not HSDPA is applied to chip dataon the respective frequency channel notified on a chip basis frombaseband control board 910 in base station (BTS) control section 900.

For convenience, FIG. 15 does not show data of frequency channels CH2and CH3. In the figure, shaded chips are chips to which HSDPA isapplied.

As shown in the figure, the HSDPA application on/off information (DP1 toDP4) is high level on chips to which HSDPA is applied, and similarly,frequency on/off information (F1 to F4) is high level when a frequencychannel is used.

As shown in FIG. 16, the HSDPA application on/off information (DP1 toDP4) and frequency on/off information (F1 to F4) is collectivelytransformed into address information of eight bits. In this case, “on”of each piece of information corresponds to “1”, while “off” correspondsto “0”.

In this way, total 256 patterns exist. For each index, ROM address isassociated with ROM data (data of limit value), and ROM data (data oflimit value) is written in ROM to generate a lookup table.

As described in FIG. 16, for example, situations {circle around (1)} to{circle around (7)} are assumed.

In consideration of each situation, the limit value is set such that agreater limit value is applied to a frequency channel that uses HSDPAthan a frequency channel that does not use HSDPA to prevent signalquality from deteriorating, and that in a case where an unused frequencychannel is present, as the number of unused frequency channels isincreased, the limit value for use in a used frequency channel isincreased, to prevent signal quality from deteriorating.

As indicated in FIG. 16, with respect to limit values L1 and L2, L2 isgreater than L1. L3 is calculated by obtaining a fraction of adenominator that is the number of frequency channels with frequencyon/off bit on and a numerator of 4, raising the fraction to the power of½, and further multiplying the resultant by L1. Similarly, L4 iscalculated by obtaining a fraction of a denominator that is the numberof frequency channels with frequency on/off bit on and a numerator of 4,raising the fraction to the power of ½, and further multiplying theresultant by L2.

FIGS. 17A and 17B are views each showing an example of effects in thecase of applying the adaptive peak limiter.

FIG. 17A is a view showing results of measurements (simulation) of errorvector magnitude that is an index to evaluate the signal quality onsamples (plotted by white circles) to which HSDPA is not applied andsamples (plotted by lozenges with black-shaded half) to which DSDPA isapplied.

In the figure, standard A is a requirement (criterion by which toevaluate samples plotted by white circles) in 3GPP R99, and standard Bis a requirement (criterion by which to evaluate samples plotted bylozenges with black-shaded half) in 3GPP R5.

Similarly, FIG. 17B shows samples measured on peak code domain error,where standard C is a requirement in 3GPP R99, and standard D is arequirement in 3GPP R5.

It is understood that the requirement of the signal quality is satisfiedin both FIGS. 17A and 17B.

FIGS. 18A and 18B are graphs each showing an example of a result ofmeasuring the degree of peak suppression of a multicarrier transmissionsignal output from the baseband signal processing LSI as illustrated inFIG. 1, using test model 1 or test model 3 of 3GPP.

As can be seen from both graphs, changes in test model do not vary theshape of the line of property of peak suppressing characteristics, anddesired peak suppression is always achieved.

Therefore, according to the present invention, it is possible tosuppress instantaneous peaks of the entire multicarrier transmissionsignal to be within a desired range in any cases, while finely adjustingthe amplitude of the transmission signal depending on situation of eachfrequency channel, and to achieve both peak suppression and assurance ofsignal quality.

The hybrid distortion compensating circuit (including a high-frequencyamplifier) shown in FIG. 19 (and in FIG. 1B) will be describedspecifically.

As described above, CDMA multicarrier communications require higherlinearity in a high-frequency power amplifier than in other mobilecommunications. Therefore, power efficiency deteriorates remarkablyunless the linearity of a power amplifier is compensated usingdistortion compensating techniques such as adaptive pre-distortion.

An input signal of a power amplifier has, for example, a bandwidth of 15MHz to 20 MHz. Accordingly, the band of a distortion ranges from about100 MHz to 200 MHz.

In order to compensate for the distortion component only by adaptivepre-distortion, it is required to perform D/A conversion on digitalsignals subjected to the pre-distortion processing with a samplingfrequency of from about 100 MHz to 200 MHz the same as the band of thedistortion component.

Further, when the adaptive pre-distortion processing is executed, sinceit is necessary for an output signal of the power amplifier to bereturned to the digital signal processing system, it is similarlyrequired to perform A/D conversion with a sampling frequency of fromabout 100 MHz to 200 MHz the same as the band of the distortioncomponent.

Furthermore, according to specifications of CDMA communication system,D/A converters and A/D converters require the resolution of from 12 bitsto 16 bits.

In current semiconductor manufacturing techniques, it is considerablydifficult to manufacture a D/A converter and A/D converter operable in arange of 100 MHz to 200 MHz with high resolution (12 bits to 16 bits)secured.

Further, if such D/A converter and A/D converter can be manufactured,power consumption will be enormous in operating. Such products goagainst distortion compensation to improve power efficiency.

Therefore, in the hybrid distortion compensating circuit in FIG. 19limits a band of a signal (input baseband signal) to which the adaptivepre-distortion processing is applied to frequencies enabling theresolution of 12 bits to 16 bits in the D/A converter and A/D converter.

Then, the feedforward distortion compensating circuit withcharacteristics accurately adjusted effectively removes distortion(distortion of high order) occurring in a band of higher frequencies bydigital signal processing.

In this way, it is possible to implement distortion compensation withextremely high precision that could not be achieved conventionally,using exiting LSI techniques.

The specific description is given below.

As illustrated in FIG. 19, the hybrid distortion compensating circuithas, as primary structural elements, adaptive pre-distortion section(digital signal processing section) 14, high-frequency power amplifier32, feedforward distortion compensating circuit (high-frequency poweranalog circuit) 30 with two input terminals TA1 and TA2, high-frequencyswitching circuit (hereinafter, simply referred to as a switchingcircuit) SW to fetch selectively either of two input signals, outputsignal and feedforward loop signal of feedforward distortioncompensating circuit 30, control/monitoring section (belonging to thedigital signal processing system), amplitude/phase/delay adjustor 51that adjusts the amplitude (gain), phase and delay of a standard signal(that is an input signal (IN) of the distortion compensating circuit)provided to input terminal TA2 of feedforward distortion compensatingcircuit 30, and sequencer 80 that controls switching of switchingcircuit SW and provides information (P1 and P2) required for causingsections to operate sequentially to the sections.

A signal path on which signals are provided and received between thedigital signal processing system and analog signal processing system isprovided with D/A converters 20 and 56, A/D converter 28 and frequencyconverting circuit. The frequency converting circuit has RF carrieroscillator 24, and mixers 22, 26 and 58 as structural elements.

As shown in the figure, feedforward distortion compensating circuit 30has input terminal TA1 to input a signal including a distortioncomponent (linear distortion component remaining without being removedby pre-distortion distortion compensation) to a main path, and inputterminal TA2 to input a standard signal that does not include distortionto the feedforward loop. In addition, the main path is a line connectinginput terminal TA1 and combiner 38.

The feedforward loop has attenuator 42 that adjusts signal amplitude,combiner 46 to separate a distortion component from a signal of the mainpath, error amplifier 48 that amplifies the amplitude of a signal of adistortion component, shifter 50 to invert a phase of an output signalof error amplifier 46, and combiner 38 to return an output signal ofshifter 50 to the main path.

The hybrid distortion compensating circuit has a hybrid structure with acombination of adaptive pre-distortion section 14 that performs adaptivepre-distortion processing on baseband digital signals and feedforwarddistortion compensating circuit 30.

However, it is impossible to simply combine both compensation schemes.It is because feedforward distortion compensation, as the nameindicates, executes distortion compensation in the order in whichsignals are input and output, while adaptive pre-distortion distortioncompensation is feedback type of distortion compensation, thus signalroutes are different therebetween, and therefore, in order to combineboth the schemes, it is required to divide both schemes into respectiveunit elements to facilitate combining both schemes, and configure againa hybrid structure.

Therefore, in the circuit in FIG. 19, feedforward distortioncompensating circuit 30 is provided with two input terminals TA1 andTA2, and thus provided with a new configuration to receive as its inputsindependently of each other an output signal (including a remainingdistortion component that cannot be removed by pre-distortion distortioncompensation) of high-frequency power amplifier 32, and a standardsignal that does not include distortion, thereby enabling a combinationof different types of distortion compensating circuits.

The distortion compensating processing in the hybrid distortioncompensation method is principally divided into two kinds of processing.

In other words, the adaptive pre-distortion distortion compensation infull-digital control removes with high stability a distortion componentof low order of the high-frequency power amplifier that is a distortioncomponent with a high level within sampling frequency band of D/Aconverters 20 and 56 and A/D converter 28.

Then, remaining high-order IM distortion component with a low level(component outside sampling frequency band) is removed by thefeedforward distortion compensating processing. In this way, it ispossible to implement the wide-band distortion compensation with highaccuracy that has not been achieved before.

An issue is that unless the precision is high in the feedforwarddistortion compensation using the analog circuit, the high-order IMdistortion component with a low level is not removed adequately thatcould not be removed by the adaptive pre-distortion distortioncompensation, and that it is not possible to achieve dramaticimprovements in precision in removing distortion that the presentinvention aims.

The distortion cancellation with high precision in feedforwarddistortion compensating circuit 30 is achieved on the assumption thattwo signals respectively input to two input terminals TA1 and TA2 are incompletely agreement with each other in input level (amplitude), phaseand delay.

Therefore, the distortion compensating circuit (hybrid distortioncompensating circuit) in FIG. 19 is provided with an adjustmentmechanism that performs adjustments so as to bring each of amplitude andothers of two signals input to feedforward distortion compensatingcircuit 30 into completely coincident with respective one another, andin this respect, the distortion compensating circuit of the presentinvention has an extremely important feature.

In other words, in the distortion compensating circuit in FIG. 19, withattention attracted to a feedback path (signal path to return a signalsubjected to feedforward distortion compensation processing to adaptivepre-distortion section 14) that is inevitable in the adaptivepre-distortion processing, using the feedback path, two input signals(signals A1 and A2 in FIG. 19) of feedforward distortion compensatingcircuit 30 and signal of feedforward loop (signal A3 in FIG. 19) arereturned to the digital signal processing system.

Then, using high-precision digital signal processing, control/monitoringsection 60 measures precisely differences (at least a difference ineither of characteristics) in amplitude (gain), initial phase andtransmission delay between two input signals of feedforward distortioncompensating circuit 30.

Next, it is preferable that the adjustor 50 for amplitude and othersadjusts at least one of the amplitude, phase and delay of a standardsignal (input signal (IN) of the distortion compensating circuit) so asto cancel the measured difference. In addition, practically, it ispreferable to adjust all the characteristics.

In this way, characteristics such as the amplitude (gain), initial phaseand transmission delay of two input signals of feedforward distortioncompensating circuit 30 are in completely good agreement with respectiveone, and conditions to perform the feedforward distortion compensationare satisfied.

In output signals of high-frequency power amplifier 33 input tofeedforward distortion compensating circuit 30, distortion with a highlevel is removed by the pre-distortion distortion compensation.

Accordingly, a distortion component with a high level is not input toerror amplifier 48 existing in the feedforward loop. It is therebypossible to set the error amplifier for a low power amplification rate,thus contributing to reduced power consumption.

After finishing the pre-distortion processing and characteristicadjustments of two signals of feedforward distortion compensatingcircuit 30, switching circuit SW outputs an output signal (signal A4 inFIG. 19) of feedforward distortion compensating circuit 30 to return tothe digital signal processing system.

Control/monitoring section 60 monitors the characteristics of feedbacksignals, and when desired precision cannot be assured in distortioncompensation, executes again sequentially the pre-distortion processingand characteristic adjustments of two signals of feedforwarddistribution compensating circuit 30. The order of signal processing iscontrolled by sequencer 80.

The primary operation (and primary states of the circuits) as describedabove is summarized as shown in FIG. 20.

That is, first, the switching circuit (SW) is switched to “d” terminalside, and adaptive pre-distortion processing is carried out (state 1,step 100).

The switching circuit (SW) is next switched to “a” terminal side.

The imbalance in the gain (amplitude), delay and phase between two inputsignals (input signal to the main path and standard signal) infeedforward distortion compensating circuit 30 is measured, and tocancel the invalance, characteristics of the standard signal is adjusted(state 2, step 102).

The switching circuit (SW) is next switched to “b” terminal side, thusshifting to state 3 to check a result of adjustment in state 2.

In state 3, a power level (leak level of the standard signal) ofcomponents of the standard signal is measured except a distortion signalin the feedforward loop (step 104). It is determined whether or not theleak level exceeds a threshold, that is, whether the leak amount isallowable (OK) or not, and at the time of NG, the processing flowreturns to step 102, while proceeding to state 4 at the time of OK (step106).

In state 4, the switching circuit (SW) is switched to “c” terminal side.Then, the frequency spectrum of a final output signal of the distortioncompensating circuit is measured, and compared with a predeterminedstandard mask pattern (spectrum emission mask pattern) to determine astate of suppression in distortion on the frequency axis (step 108).

As a result of the determination, when the frequency spectrum issuppressed to be within an allowable range (step 110), the processingflow returns to step 108 to continue monitoring, while returning to step100 when the spectrum is not suppressed (step 110) to execute theabove-mentioned processing sequentially.

FIGS. 21A to 21D respectively show frequency spectra of the input signal(the number of carrier is “3”), pre-distortion signal, standard signalin the feed forward distortion compensation and output signal in thecircuit in FIG. 19.

As can be seen from the figures, according to the present invention, itis possible to perform distortion compensation with high precision in awide range.

Thus, the hybrid distortion compensating circuit in FIG. 19 has adaptivepre-distortion section 14 that provides an input digital signal withdistortion with inverse characteristics to non-linear characteristics ofthe power amplifier, and feedforward distortion compensating circuit 30that compensates for a distortion component, which cannot be compensatedin adaptive pre-distortion section 14, by feedforward loop, wherefeedforward distortion compensating circuit 30 has two signal inputterminals TA1 and TA2 enabling two signals to be input separately, asignal subjected to the adaptive pre-distortion processing inpre-distortion section 14 is input to one of the signal input terminals,TA1, while a standard signal is input to the other one of the signalinput terminals, TA2, and the standard signal corresponds to an inputdigital signal prior to the pre-distortion processing in pre-distortionsection 14, and thus connects both circuits in a manner capable ofdrawing maximum characteristics of each circuit.

In other words, the distortion compensating circuit in FIG. 19 is a newdistortion compensating circuit in full-digital control having a circuitstructure connecting a digital signal processing circuit andhigh-frequency power analog circuit via a signal path containing D/Aconverters and A/D converter.

The distortion compensating circuit preferably performs processing offollowing items {circle around (1)} to {circle around (5)} and obtainseffectiveness as described below.

{circle around (1)} The adaptive pre-distortion processing is performedin digital signal processing.

Since the pre-distortion is implemented by digital signal processing, itis possible to perform the processing with higher precision than inanalog pre-distortion.

{circle around (2)} A high-frequency analog signal is fetched fromfeedforward distortion compensating circuit 30, the fetched analogsignal is converted into a digital signal, desired characteristics ofthe digital signal are measured with extremely high precision using theadvance digital signal processing such as frequency spectral analysis,and the measurement result is used as a base for controlling andmonitoring the entire circuit.

In other words, since control and monitoring is performed using as abase high-precision data incomparably than the analog signal processing,significant increases are obtained in both the adaptive pre-distortionprocessing function and feedforward distortion compensating function,and the distortion compensation capability is dramatically improved.

{circle around (3)} The distortion compensating processing is dividedinto a plurality of stages that are controlled sequentially.

Although communication environments vary every instant, it is regardedcharacteristics of a signal do not vary in a short term. Focusing onthis respect, by executing a plurality of stages sequentially accordingto predetermined procedures, it is possible to execute the distortioncompensation processing in digital control reasonably.

{circle around (4)} The plurality of stages includes, for example, afirst stage of performing the adaptive pre-distortion processing, asecond stage of adjusting and matching characteristics such as theamplitude, phase and delay amount of two input signals independentlyinput to feedforward distortion compensating circuit 30, that is, aninput signal to the main path that contains non-linear distortion andstandard signal that does not contain non-linear distortion (signal tobe input to the feedforward loop), a third stage of checking a result ofthe adjustment of the second stage, and a fourth stage of monitoringcharacteristics of a signal subjected to the feedforward distortioncompensation.

Since the adjustment is always carried out precisely to matchcharacteristics of two independent input signals of feedforwarddistortion compensating circuit 30 with each other, it is possible toeliminate adverse effects caused by the presence of adaptivepre-distortion section 14 in a first half on the feedforward distortioncompensation. Accordingly, the precision is assured in each of adaptivepre-distortion and feedforward distortion compensation, and the synergyof both processing enables remarkably improved distortion compensatingperformance.

That is, an adaptive pre-distortion distortion compensating circuit indigital control is not able to remove a high-order TM distortioncomponent (inter-modulation distortion component) with a low levelspread out of a band of the sampling frequency of an A/D converter andD/A converter.

However, it is possible to remove a low-order distortion component of apower amplifier that is a high-level distortion component in a bandwithin the sampling frequency with high stability. Then, remaininghigh-order IM distortion component with a low level is removedeffectively in the high-precision feedforward distortion compensationprocessing, thus implementing the distortion compensation on signals ina wide band with stability and with high accuracy.

Further, since the distortion is suppressed accurately, it is possibleto decrease the gain of an error amplifier provided in the feedforwardloop in feedforward distortion compensating circuit 300, resulting inreduced power consumption.

{circle around (5)} Through the first to third stages as describedabove, when a series of adjustments is finished on the entire distortioncompensating circuit, the processing flow proceeds to a monitoring stage(fourth stage). As long as the distortion is suppressed to be within apredetermined range, adjustments are not carried out such as adaptiveadjustments of pre-distortion characteristics and adjustments ofcharacteristics of input signals of feedforward distortion compensatingcircuit 300, and characteristics of these circuit are fixed during thisperiod. Accordingly, also in this respect, it is possible to reducepower consumption, as distinct from an analog circuit that alwaysperforms adaptive control.

{circle around (6)} Further, since it is possible to use digital signalprocessing functions (such as correlation detection and powermeasurement) with which recent mobile communication apparatuses aregenerally provided, implementing the distortion compensating method ofthe present invention is relatively easy and has great value inpractical use.

As shown in FIGS. 1A and 1B, by combining techniques of the presentinvention, it is possible to obtain excellent advantages that have notbeen obtained before.

That is, by the technique of suppressing a peak of a multicarriertransmission signal, it is possible to suppress instantaneous peaks onthe entire multicarrier transmission signal to be within specificationsin any cases, and it is thereby possible to prevent power efficiency ina high-frequency power amplifier disposed subsequently fromdeteriorating.

In other words, when the peak suppression of a multicarrier signal isnot sufficient, as the need of a great margin, it is required to providea dynamic range in around region A2 in FIG. 22 showing input/outputcharacteristics of the high-frequency amplifier. However, when the peaksuppression of a multicarrier signal is sufficient, it is possible tooperate the high-frequency amplifier in around Al, and it is thuspossible to prevent power efficiency in the high-frequency poweramplifier from deteriorating.

Further, when HSDPA in W-CDMA is applied by using the technique of theadaptive peak limiter, that is, when stricter control is required sincemodulation schemes are switched adaptively, it is possible to achieveboth the peak limit and signal quality.

Furthermore, since the distortion compensation with high accuracy iscarried out in the hybrid distortion compensating circuit, it ispossible to assure the quality with a desired level in transmissionsignal.

In this way, it is possible to implement next-generation mobilecommunications conforming to specifications of 3GPP.

While the W-CDMA communication system is described as an example in theforegoing, the present invention is applicable to other communicationsystems. For example, the peak limiter of the present invention isapplicable to other CDMA communication systems that support high speedpacket transmission.

Thus, in the present invention, with respect to techniques of limiting apeak and compensating for a distortion that are inevitable intransmission circuitry in CDMA systems (including the W-CDMA system), bytaking realistic measures in consideration of implementing high speeddata packet transmission and others, it is possible to achieve, forexample, High Speed Downlink Packet Access (HSDPA) in the W-CDMA system,while clearing up strict constrains imposed on mobile communicationapparatuses.

The present invention is not limited to the above described embodiments,and various variations and modifications may be possible withoutdeparting from the scope of the present invention.

This application is based on the Japanese Patent Application No.2002-224221 filed on Jul. 31, 2002, entire content of which is expresslyincorporated by reference herein.

1. A method of suppressing a peak of a multicarrier transmission signalin a transmission system where filtering processing is performed on eachof baseband signals respectively corresponding to a plurality offrequency channels using a filter, the signals subjected to thefiltering processing are each multiplied by a predetermined carrier tobe single-carrier signals, and the single-carrier signals are combinedto obtain a multicarrier transmission signal, comprising the steps of:branching each of the baseband signals from a regular signal processingroute, performing filtering processing on each of the baseband signalsbranched, multiplying each of the baseband signals branched by the samecarrier as the predetermined carrier at the same timing as inmultiplication by the predetermined carrier, combining the signalsobtained, and thereby obtaining a multicarrier signal for use incalculating a correction value for peak suppression; detecting aninstantaneous peak of the multicarrier signal for use in calculating thecorrection value, and based on the detection result, obtaining thecorrection value for peak suppression; and multiplying each of thebaseband signals on the regular signal processing route by thecorrection value to perform correction for peak suppression.
 2. Themethod of suppressing a peak of a multicarrier transmission signalaccording to claim 1, wherein the step of obtaining the correction valuefor peak suppression comprises the steps of: detecting a peak value foreach of a predetermined items of data; and calculating the correctionvalue to output, when the current peak value detected exceeds apredetermined threshold and is larger than a last peak value, so thatthe correction value adapts to the current peak value, while outputtinga correction value corresponding to a peak value obtained immediatelybefore the peak value starts decreasing without updating, when the peakvalue decreases continuously the number of times that does not exceed apredetermined number of times.
 3. A multicarrier transmission signalgenerating circuit with peak suppressing function, comprising: a regularsignal processing route for branching each of baseband signalscorresponding to each of frequency channels to multicarrier-transmit totwo signal sequences, delaying each of baseband signals in one signalsequence in a delayer, multiplying each of the signals by a correctionvalue for peak suppression in a multiplier, performing n-time (n is aninteger of two or more) interpolation processing on each of the signalsmultiplied by the correction value, performing filtering processing onthe signals using a filter, multiplying each of the signals by a carrierto obtain single-carrier signals, and combining the single-carriersignals to output a multicarrier transmission signal; and a correctionvalue generating route for performing on each of baseband signals in theother signal sequence substantially the same processing at substantiallythe same timing as the n-time interpolation processing, the filteringprocessing, and processing of multiplying by the carrier to obtain asingle-carrier signal in the regular signal processing route, therebyobtaining a multicarrier signal for use in calculating the correctionvalue, detecting an instantaneous peak of the multicarrier signal foruse in calculating the correction value, and obtaining the correctionvalue for peak suppression based on the detection value to provide tothe multiplier in the regular signal processing route.
 4. Themulticarrier signal generating circuit with peak suppressing functionaccording to claim 3, wherein timing of multiplication by the carrier inthe regular signal processing route is controlled to be timing delayedfrom timing of multiplication by the carrier in the correction valuegenerating route by a time required for calculating the correction valueand a time corresponding to a group delay of signals associated with thefiltering.